1. Field of the Invention
The present invention relates to a disk selection method used with a RAID device, a disk selection program for the method, a RAID control device, a RAID system, and a disk device for the system.
2. Description of the Related Art
There are well known RAID systems for storing data by distributing the data on a plurality of disk devices, and for storing the same data on a plurality of disk devices.
In the RAID system at a RAID1 level for storing the same data on a plurality of disk devices, it is necessary to select a disk device for a read such that data read time can be shortened.
For example, the patent document 1 describes calculating an average value of an access address in the disk array device having a mirroring configuration, comparing a read request address with the calculated address average value, and determining from which disk device the data is to be read.
FIG. 8 is an explanatory view of the conventional disk selection method. Assume that the minimum and maximum values of the logical block address LBA of mirrored disk devices 21 and 22 are “0” and MAX respectively.
When the average value of the addresses of a plurality of read requests is boundary LBA, assume that the disk device for reading data, a disk device for reading data if an address lower than a boundary LBA is specified is defined as the early shift disk device 21, and a disk device for reading data if an address of or over the boundary LBA is specified is defined as the late shift disk device 22.
As described above, by setting the boundary LBA as an average value of the address of a read request, and switching a read destination disk device, the seek distance of the head of each disk device can be reduced.
When one of the RAID devices becomes faulty, it is necessary to copy the same data as the faulty disk device to another disk device. This process is referred to as a rebuilding process.
FIG. 9 is an explanatory view of the seek (travel) distances of a normal disk device 23 and a rebuild destination disk device 24. FIG. 9 shows the case in which the normal disk device 23 is a late shift disk device, and the rebuild destination disk device 24 is an early shift disk device.
Assume that the boundary LBA as an average value of a plurality of read addresses specified by a host computer is a substantially the central address between the maximum value MAX and the minimum value 0 of the LBA of the disk device, and the address of the area a during rebuild is close to the MAX of the LBA.
When the read address requested by the host computer is higher than the boundary LBA, data is read from the normal disk device 23.
In this case, when the read address is a little higher than the boundary LBA, the seek distance for the travel of the head from the area a during rebuild to the position of the read address is the maximum seek distance of the normal disk device 23.
When the read address requested by the host computer is a little lower than the boundary LBA, data is read from the rebuild destination disk device 24 as an early shift disk device.
In this case, when the read address is a little lower than the boundary LBA, the seek distance of the travel of the head from the area a for rebuild to the position of the read address is the minimum seek distance. When the read address is the minimum value of “0”, the seek distance of a moved head from the area a during rebuild to the position is the maximum seek distance.
Depending on whether or not the above-mentioned read address is equal to or higher than the boundary LBA, the seek distance of a head for performing a normal read during rebuild is long in the disk selection method for switching the read destination disk device, thereby causing the problem of speeding down the response to the read request of the host computer.
[Patent Document 1] Japanese Published Patent Application No. 2002-163075